To make the burger "meat," press Sauté and add the olive oil to the Instant Pot. Once the oil is hot but not smoking, add the onion and sauté until softened, about 5 minutes. Press Cancel to stop the cooking cycle.
Stir in the garlic powder, paprika, cayenne, cumin, and black pepper while the pot is still hot. Add the green lentils and water, and stir to make sure the lentils are covered in the liquid for even cooking. Secure the lid and move the steam release valve to Sealing. Select Manual/Pressure Cook to cook on high pressure for 5 minutes.
While the lentils are cooking make the special sauce. Drain and rinse the cashews, then add them to a blender with the water, vinegar, maple syrup, tomato paste, mustard, onion, powder, and salt. Blend until very smooth and set aside.
White the cooking cycle on the burger "meat" is complete, let the pressure naturally release for 10 minutes, them move the steam release valve to Venting to release any remaining pressure. When the floating device valve drops, remove the lid and stir in the salt and chopped walnuts.
Fill a bowl with chopped lettuce, the burger "meat," tomatoes, green onions, pickles, and cheese. Drizzle plenty of the special sauce over the top before serving. Store leftovers in three separate airtight containers- - for the dressing, the burger "meat," and the vegetables -- in the fridge for 1 week.